The machine is a AMD Phenom(tm) 9550 2.2GHz, 64bit Quad-Core Processor and this is on Arch Linux with .vimrc moved (so Arch's vim defaults are used). Both vim and gvim are affected.

If I copy paste the lines defining variables 5 times, resulting in about 48 lines, the delay is 3 seconds long. Increasing to over 400 lines causes the same delay which makes me assume there's a timeout that is being reached.

If your Vim version (recent ones with "huge" features) supports the :syntime command, you can dive deeper; cp. :help :syntime.

This may turn up a pattern that is responsible for the slowness; you'd then contact the syntax plugin author (whose address / link to issue tracker you'll find in the script's header: $VIMRUNTIME/syntax/python.vim.